In a variety of environments, signals must be transmitted between diverse sources and circuitry, while maintaining electrical (i.e., galvanic) isolation between the sources and the circuitry. A signal isolator is a device or circuit for transmitting logic signals across an isolation barrier. A signal isolator may use optical or magnetic isolation to transmit an input signal from one side of the isolation barrier to the other. Each side of the isolation barrier may operate at different voltages, have a different supply voltages and have separate, isolated grounds. Micro-transformers have been used as signal isolators in combination with either signal level detectors or edge detectors sending multiple pulses across the transformer upon receiving a positive input signal and a single pulse upon receipt of a negative input signal.
FIG. 1 illustrates a single-bit signal isolator manufactured by Analog Devices, Incorporated. The isolator includes edge detection circuits 101 and 103 for detecting rising and falling edges in the input signal. Edge detection circuit 101 outputs two pulses when a rising edge is detected and edge detection circuit 103 outputs a single pulse when a falling edge is detected. The output of the edge detection circuits 101 and 103 is input into an OR gate 104. The combined signals are then transferred across a micro-transformer 105. A receiver 106 then reconstructs the input signal by determining if the received signal has two pulses or one pulse.
The signals transferred across the isolation barrier typically have to be of a sufficient voltage to overcome noise in the system due to transient signals. The isolators are used in many noisy industry environments and there could be high common mode transients between the two grounds to be isolated. For a single high end receiver, the noise generated at the receiver by the common mode transients should be smaller than the receiver threshold and the signal generated by the transformer driver should be large enough to be reliably detected in the presence of noise.
FIG. 2 illustrates the a signal transmitted across an isolation barrier using micro-transformers and using the signal level detector technique which transmits a positive pulse upon detection of a rising edge and transmits a negative pulse upon detection of a falling edge in the input signal. As seen in FIG. 2, when the primary coil of the micro-transformer receives a positive pulse the secondary coil receives a positive pulse, however, there is a negative tail at the end of the received pulse. Likewise, when the primary coil receives a negative pulse the secondary coil receives a negative pulse, however, there is a positive tail at the end of the received pulse. The tail end of the pulses received on the secondary coil are caused by a buildup of current in the primary coil through its large serial resistance.
Because of the noise inherent in the digital isolator system, the transmitters must transmit signals using a large enough voltage to be detected over the noise. Accordingly, there is a need for a isolation system which reduces the amount of noise in the digital isolator system.